Method and apparatus for compacting soil



Nov. 1 8, 1969 c b 3,478,656

METHOD AND APPARATUS FOR COMPACTING SOIL Filed July 24, 1967 4Sheets-Sheet 1 JOHN K. M DONALD INVE/VTUR BY BUG/(HORN, BLORE, KLAROU/ST8 SPAR/(MN AT T OR/VE K5 1969 J. K. M DONALD METHOD AND APPARATUS FORCOMPACTING SOIL 4 Sheets-Sheet 2 Filed July 24, 1967 JOHN BUCKHOR/V,BLO/FE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS N 3, 1959 J. K. M DONALD METHODAND APPARATUS FOR COMPACTING SOIL 4 Sheets-Sheet 5 Filed July 24, 1967llla Nov. 18, 1969 J. KIM DONALD 3,478,655

METHOD AND APPARATUS FOR COMPACTING SOIL Filed July 24, 1967 4Sheets-Sheet 4 Ill/[582233521 u "I' -2lZ JOHN K. MDONALD //VVE/V7'0/?fig; J8 5) 2J0 BUCKHOR/V, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS UnitedStates Patent ABSTRACT OF THE DISCLOSURE I a The present applicationdiscloses a method and means for compacting soils and other particulatematerials. Ac-

cording to an illustrative embodiment of the method a soil is compactedby applying surface pressure to a zone of the soil to place the sameunder compression. While the zone is under compression, compressed airor other gas is introduced into the soil under compression to weaken thesame. The combinedeffects of the continued surface pressure andincreased internal gas pressure within the soil collapses, and thusdensifies, the soil to a degree not obtainable by the application of thesurface pressure alone. Air or other gas within the soil zone ispermitted to bleed therefrom, either through the surrounding soil duringthe entire operation, or upwardly through the densified soil zonefollowing its collapse and release .of the surface 1 surface forpressing on the material to be compacted such as embodied in an earthtamping foot or earth com- .pacting roller, an external source of apressurized gas,

and a device for injecting the gas under a controlled pressure into thematerial. The gas can be injected into the material through smallopenings in the material contacting surface of the compaction device,through injections arranged'peripherally about the contact surfaceor bymeans ofa hood placed over a surface-portion of the zone and into whichthe pressurized gas is pumped.

Applications of the method include the densification of any gaspermeable particulate material including, for example, soils, foundrysand, powdered metals, ceramic powders and dental materials.

i :2 BACKGROUND OF THE INVENTION U I i Field of the inventionDescription of the prior art Commonly known methods of compacting soilsand othermaterials include (1) application of surface pressures to thematerial to press the individual particles of 'material more closely-together, (2) vibration of the. material to effect a more compactrearrangement of the individual particles, (3) application ofextraordinary sur- ,face pressuresto the material so as to crushtheoriginal 1 particles into smaller, more closely inter-fittingparticles, 4) acombination of any of the foregoing three methods,

and (5)-jetting :orinundation of granular soilswith anincompressiblefiuid, usually water.

.. In the manufacture of bricks and similar ..products,wet ,clays,=which are relatively impervious to air, have been worked in :a vacuum,atmosphere prior to extruding or compacting the material to; remove air.therefrom and thereby prevent the formation of air pocketsin thefinished product. i

-In the fields of structural and soils engineering, the

effective density ofa soil mass and thus its ability to 31,478,656Patented Nov. 18, 1969 "ice support superimposed loads, has beenincreased through the injection of a fluent material such as a groutinto the air voids of the mass to create a load-bearing region upon thehardening of the grout. In a related method, sodium silicate has beenmixed with soil, and then the mass has been chemically stabilizedthrough the injection of car- 'bon dioxide, which reacts with the sodiumsilicate to form a stiff glue.

Soils such as quicksand, which are inherently unstable because of highinternal water pressures, have been stabilized through the removal ofexcess Water therefrom by drainage or pumping.

The present invention, however, employs a new technique to thecompaction of soils and other materials having some degree of airpermeability which gives a greater compactive effect than can beobtained by using the aforementioned surface compaction methods alone,and without the necesstiy of introducing artificial hardening agents orexcess water into the material.

SUMMARY OF THE INVENTION The present invention involves the injection ofcompressed air or other nonreactive gas into an air permeable soil orother particulate material while a compaction device is pressing on thematerial surface. Somehow the combination of the resulting increasedinternal gas pressures within the material and the simultaneous surfacepressure weakens the soil to such an extent that it collapses, orcompacts, to a greater degree than would result from the application ofeither pressure alone. It has been found in working with soils that theapplied gas pressure necessary to achieve the optimum compactive effectvaries with the applied surface pressure, the type of soil and the Watercontent of the soil. In general, the optimum applied gas pressure for agiven surface pressure must be determined empirically on a case by casebasis.

Principal objects of the invention are to provide:

(1) a new method of compacting soils and other particulate materialsutilizing the injection of air or other gas pressure into the materialas one of the necessary steps of the method;

(2) a new method as aforesaid which gives a greater compactive effectthan can be obtained using prior. ord inary compaction methods relyingsolely on applied surface pressures or vibration;

-(3) an improved compaction method as. aforesaid which is substantiallyas simple and economical to perform as prior known methods;

(4) a compaction-method as aforesaid which utilizes (5) apparatus forcarrying out the aforesaid method;

- and Y i (6) apparatus as aforesaid which is eflicient, practical,economical to manufacture and potentially adaptable to existingcompaction equipment.

Although the method and apparatus as described here- 'inafter willreferspecifically to the compaction ofasoil utilizing the'injection ofair under pressure, reference to soil and air is illustrative only. Itis to; be understood that the description applies to other particulatematerials "and' to other compressible'fluids as well as to soil and to hir. 1 e5;

BRIEF DESCRIPTION. OF THE DRAWINGS I FIG. 1 is a schematic sideelevational View, partly in section, showing a laboratory apparatus forperforming the method of the present invention;

FIG. 2 is a diagram of a loose mass of soil;

FIG. 4 is a diagram on an enlarged scale of some of the soil particlesof FIG. 2, illustrating the theory of the present invention;

FIGS. 5 through 11 are schematic sectional views of various forms ofcompaction feet in accordance with the apparatus of the presentinvention;

FIG. 12 is a schematic sectional view of a compaction rollerincorporating compaction feet as shown in FIG. 11;

FIG. 13 is a schematic sectional view of a modified form of compactionroller for carrying out the method of the present invention;

FIG. 14 is a schematic sectional view of another modification ofcompaction roller in accordance with the invention;

FIG. 15 is a schematic end view of a modified form of compaction rollerin accordance with the invention;

FIG. 16 is a schematic side view of the compaction roller of FIG. 15;

FIG. 17 is a schematic vertical sectional view of still anothermodification of compaction roller in accordance with the invention;

FIG. 18 is a partial sectional view taken along the line 18-18 of FIG.17;

FIG. 19 is a sectional view through a peripheral portion of a rollersimilar to that of FIG. 17 but modified to exclude tamping feet;

FIG. 20 is a schematic side elevational view of a vibratory tampingdevice in accordance with the invention; and

FIG. 21 is a schematic side elevational view of a vehicle mountedmultiple shoe vibrator in accordance with the invention.

DETAILED DESCRIPTION Description of method With reference to thedrawings, FIG. 1 diagrams a laboratory apparatus 10 which has been usedto demonstrate the effectiveness of the method of the present invention.The apparatus includes a generally rectangular housing 12 which supportsfrom an overhead portion 14 a suspended hydraulic ram 16 including apiston rod 18 carrying at its lower end a compaction foot 20. The ram isconnected by a suitable hose 22 to a hydraulic pump 24. A suitable valve26 in hose 22 between the pump and the hydraulic ram controls theapplied pressure of the compaction foot, such pressure being indicatedon a pressure gauge 28.

Compaction foot 20 is shown in section and includes a lower, soilcontacting surface 30' which has a recessed central portion 32 whichreceives a perforate insert 34. Compressed air can be introduced intothe perforate insert through an internal foot passage 36 which connectsat an upper surface of the foot with an air hose 38 leading to a tank 40of air under pressure. Air hose 38 includes an air pressure regulatingvalve 42, and tank 40 is equipped with an air pressure gauge 44.

Soil 46 to be compacted is placed within the lower end of the housing.With the compaction foot in slight contact with the soil, a sensing arm48 of a penetration gauge 50 is placed in contact with an upper surfaceportion of the compaction foot. The gauge is mounted on the end of ahorizontally extending arm 52 forming part of a penetration gaugesupport stand 54.

With the foregoing apparatus the effectiveness of the present method isdemonstrated as follows: a constant predetermined mechanical contactpressure is applied by the compaction foot to the soil surface throughproper adjustment of the regulating valve 26. The compaction foot ispermitted to densify the soil 46 under the sole influence of thepredetermined surface contact pressure of foot 20 until such footpressure, of itself, is incapable of effecting any further substantialdensification of the soil, as indicated by the penetration gauge. Atthis point, and while still applying predetermined foot pressure,compressed air is introduced into the soil through foot insert 34 undera pressure which is sufiicient to cause the air to penetrate the soilwithout blowing away any of the surface soil surrounding the foot. Withthe introduction of the compressed air, a substantial additionalpenetration of the foot beyond that obtained with the application offoot contact pressure alone will be observed on the penetration gauge.Thereafter, when the air pressure is relieved and the compaction footwithdrawn, the densified soil remains substantially as dense as its mostdensified condition prior to withdrawal of the foot.

Similar results are obtained using various sequences of events such asapplying air pressure prior to mechanical pressure or simultaneouslytherewith. Similarly, the air pressure may be released at variousstages, or modulated.

Soil density tests have confirmed that under the foregoing conditionsthe soil has a higher retained density after the application ofcompressed air than the same soil has with the application of thesurface contact pressure alone. For example, in an experiment ofparticular interest using the illustrated laboratory apparatus and anapplied surface contact pressure of 100 p.s.i., the dial gauge readingwas .367 just prior to the application of air, whereas upon theapplication of compressed air at 34 p.s.i., the dial penetration gaugerecorded a rapid penetration and a reading of .910, or an additionalpenetration of over /2 inch out of an original compressed depth of lessthan 3 inches.

The applied air pressure requried to effect the additional compactionvaries with the type of soil, the soil moisture content, and the appliedmechanical contact pressures of the compaction device. With some onlyslightly porous soils it has been found that the air pressure mustexceed the soil contact pressure applied by the compaction foot. Inother soils, for example sand, the foregoing compactive effect has beenobserved using air at an applied pressure of less than one pound persquare inch. Still other, highly cohesive soils have been found to beunresponsive to air applied at pressures up to pounds per square inch.In general, however, clay soils have been found to be least responsiveto the effect of air pressure, sandy soils the most responsive, and siltsoils intermediate in response to applied air pressure.

Theory Referring to FIGS. 2 and 3, which diagram a loose soil mass and adense soil mass, respectively, the compaction of a soil mass or mass ofany other particulate material requires a rearrangement of the particles60 of the mass to reduce the percentage volume of voids V existingbetween the particles in the mass. These voids are usually filled partlywith air and party with moisture. When particles are shifted to reducethe volume of voids, air is forced from the voids and bled from themass. Water present in the voids remains, however, since it isincompressible and cannot readily be removed therefrom through theapplication of pressure. Thus the degree of water saturation of thevoids is a limiting factor with respect to the potential of the mass fordensification.

FIG. 4 diagrams four particles 60A, 60B, 60C and 60D of the loose massof particles 60 shown in FIG. 3, and illustrates the path of particle60A as it shifts into dashed line position 60A within a portion of thevoid V occupied initially by air a to form a more compact arrangementwith the surrounding particles. It will also be noted that water w alsooccupies a small portion of void V.

Particle 60A moves into void V by sliding over the surface of particle60B. This demonstrates that a soil mass is densified by intergranularmovement in shear rather than in compression, although the shearstresses are usually generated by external mechanical compressive forcesacting at the surface of the mass. However, before particle 60A willslide into void V, the particles frictional resistance to sliding overparticle 60B, that is, its resistance to shear, must be overcome. Thisfrictional resistance is dependent on the normal contact pressure P,(intergranular pressure) acting between particle 60A and particle 60B.The greater such contact pressure, the greater will be the particlesresistance to shear. The contact pressure P however, is increased by anyapplied external pressure P increased by pressure P created by surfacetention in water w, but decreased by pore air pressure P acting in thevoids between particles 60A and 60B. Pore air or gas pressure is thepressure applied to the particles by air or other gases acting withinthe voids between particles. I Normally pore air pressure P in a soilwould be close to atmospheric pressure. However, in the present methodit is theorized that the injection of air into the soil mass temporarilyincreases the pore air pressure P, to such an extent that it tends toforce the particles apart, thereby reducing the contact pressure Pbetween the particles and thus their resistance to shear, so that theparticles can slide more readily over one another into more compactarrangements upon the continued application of external mechanicalcontact pressure. To express the phenomenon somewhat differently, theair appears to act as a sort of lubricant between the particles totemporarily weaken the overall resistance of the entire mass to shear.While the mass is thus temporarily weakened, a given applied mechanicalcontact pressure is able to more completely shift the particles of themass into more compact arrangements than would be the case if the samecontact pressure were applied with the mass in its normal condition.

Pursuant to the foregoing theory and experience, both air pressure andmechanical contact pressure should be applied, and the contact pressureshould be maintained throughout the collapse of the mass to achieve themaximum compactive effect. If air injection occurs without theapplication of contact pressure, the soil mass will be merely puffed up,orloosened, by the .air, and pore air pressures eventually will belowered to normal or near normal by the bleeding off of air through themass. It is desirable to maintain the mechanical contact pressure on agiven mass until the end of the air injection period to minimize thepossibility of the air having a subsequent loosenng effect on the massfollowing its densification. Air which is not injected under excessivepressures for the type of soil under compaction does not appear to havea loosening effect on the soil after the compaction foot is released. Itis thought that this is due, to the natural ability of porous sandysoils to bleed off the compressed air during and following thecompaction process fast enough to prevent loosening, and the naturalability of morecohesive soils to withstand loosening while a slow Ibleeding off of the excess air takes place.

The foregoing theory appears to explain why a very porous soil such asdry sand is more responsive to the injection of air than a less poroussoil such as a silt or clayrAir is able to penetrate the porous soilmore readily and more completely to exert its friction-reducing in--:fluence.

The theory would also appear to explain why, in general, a wet soil isless responsive to the effects of air than the same soil when dry. Whenthe voids of the soil mass are filled or nearly filled with water as isthe case with a wet soil, the water prevents or at. least restricts thepassage of air into the mass and reduces the effective surface area ofthe particles against which the pore air pressure can act.

The .theory furthermore explains why a cohesive soil such as a wet clayis less responsive to air injection: than i a soilsuch as dry sandhaving no cohesion, The cohesive forces tending to hold "soil particlestogether, are .'independent of the frictionally resistive forces of vthe same .2 particles and are also independent of.the normal contactpressure acting between the particles. Thus a lessening of the normalcontact pressure by increasing pore air pressure has no effect on thecohesiveness of the particles. .If the shear stresses induced in themassare not sufiicient to overcome the cohesiorrofthe mass, then themass will Apparatus carrying out the method With reference to FIGS. 5through 8, several compaction foot configurations are shown, eachincorporating a means for injecting air under pressure into the soil ator adjacent to the soil contacting surface of the pressure foot.

More specifically, FIG. 5 discloses a compaction foot having a soilcontacting lower surface 72 and a hollow interior defining an airchamber 74 in communication witha source of compressed air (not shown).A series of straight passages 76 and tapered passages 78, 79 extendthrough the bottom of the foot and into communication with the soil overthe area of soil contacting surface 72. Of course, the passages couldall be straight or all be tapered in either direction shown, as desired.

FIG. 6 shows another form of compaction foot 80 having a soil contactingbottom surface 82 in contact with the soil 84. Bottom surface 82 has arecess 86 occupying most of the surface area of the foot and defining anair chamber in communication with a source of air pressure (not shown)through a central passage 88. Recess 86 is separated from the soil by aninsert member 90 which is pervious to air but not to soil particles andwhich serves as a means for disbursing air into the soil over a widearea. Insert member 90 may be made of sintered metal or porous ceramic.

FIG. 7 discloses a compaction foot 92 having a recess 94 in the soilcontacting surface 96 of the foot, with the recess taking up arelatively small area of such surface. Foot 92, like compaction foot 80,has a pervious insert 98 to disburse air from the recess into theadjacent soil.

FIG. 8 discloses a compaction foot 100 having a soil contacting surface102 with an annular rim 104 at its periphery. Surface 102, like thecorresponding surface of the foot 92 in FIG. 7, has a recess 106 with apervious insert 108 and a central passage 110 connecting the recess to asource of air pressure. In certain soils, the recess below insert 108will remain full of pervious compacted soil which is well dried by thepassage of air therethrough and which will thus protect the relativelyexpensive insert 108 against wear.

The compaction feet of FIGS. 7 and 8 are believed to provide moreeffective air penetration of the-soil than the compaction feet of FIGS.5 and 6 because air entering the soil through the feet of FIGS. 7 and 8must travel a greater distance, as indicated by the arrows 112 of FIGS.7 and 114 of FIG. 8, before reaching soil areas of low pressure thandoes pressurized air entering the soil through the feet of FIGS. 5' and6, as indicated by the arrow 116 of FIG. 6. Air under high pressureentering the soil through the feet of FIGS. 7 and 8 will graduallytravel from regions of the soil under high compaction pressures toregions of gradually decreasing compaction pressures as the air pressureitself decreases. However, in the foot of FIG. 6, air enters the. soilunder high pressure at points closely adjacent surrounding regions ofthe'soil near the soil surface under very low pressure, and thereforethe air under high pressure, seeking regions of lower pressure, is morelikely to flow to these low pressure soil regions near the soil surfaceand actually blow away surface soil surrounding the compaction foot thanthe foot 7 the foot and sealed at 132. As the foot approaches the soil,the shell makes contact first and progressively collapses, trapping airin the cavity defined by the soil surface, the shell and the lowersurface 134 of the compaction foot. The descending foot then compressesthe air and forces it into the soil directly ahead of the foot itself.

All of the foregoing described foot configurations could be applied toeither a simple single or multiple tamping foot device manually operatedor mounted on a vehicle, or they could be mounted as so-calledsheepsfeet on a footed compaction roller, with or without a vibrationsgenerating means in conjunction with the tamper or roller. However, whenused on compaction rollers it is especially important that means heprovided for applying the compressed air only when each foot is inproximity to the soil surface to prevent blowing away of surface soiland to save air.

FIGS. 10 and 11 disclose two different valving arrangements operative inresponse to soil contact pressure to admit air into a tamping foot onlywhen the foot is in contact with the soil. Both arrangements areespecially suited for use when the foot is provided on the periphery ofa compaction roller. In such an arrangement the roller drum itself maybe used as an air pressure accumulator. In FIG. 10 a tamping foot 136projects from the peripheral surface of a tamping roller 138 and issurrounded in spaced relationship by an outwardly biased but collapsiblebellows structure 140. The outer end portion of the bellows mounts oneend of a series of valve actuating rods 142 which slide in openings 143in the roller drum. A series of air passages 145 in the drum surroundingthe foot are normally closed by valve members 146, pivoted at 147, whenthe foot is out of contact with the soil. However, as the foot begins topenetrate the soil, the bellows starts to collapse, causing rods 142 toslide inwardly of the drum to contact valve members 146 and unseat them,thereby enabling air within the drum to enter the bellows and the soilsurrounding the foot. As the foot leaves the soil, the bellows willexpand, causing rods 142 to retract and valve members 146 to reseatthemselves in the air passages.

In FIG. 11, a compaction foot 150 has a hollow interior 151 with acentral opening 152 in the soil contacting surface 153 of the foot. Theinterior is in communication with the pressurized interior 155 of acompaction drum 156 through a valve port 157 in the drum. However, acone-shaped inner end 158 of a plunger-type valve member 159 normallycloses port 157 through the action of a compression spring 160 whichsurrounds the member between a shoulder portion 161 of an enlarged head162 of the member and the outer surface of the roller drum. The head 162extends outwardly beyond the soil contacting surface of the foot untilthe foot comes into contact with the soil, forcing the valve memberinwardly against the force of spring 160 to open valve port 157, therebyinjecting air from the drum into the soil.

As shown in FIG. 12, a frame portion 164 of the compaction roller whichincludes drum 156 and several of the compaction feet 150 of FIG. 11, isequipped with a cam 166 which contacts head 162 of each valve member 159when the associated foot is well out of contact with the soil, therebyopening the valve port so that air will blow dirt from the interior ofeach foot.

As mentioned with respect to FIGS. 10 and 11, where the tamping feetform part of a compaction roller, the entire interior of the roller drummay be used as an air pressure accumulator to supply pressure to thevarious feet when valves are actuated. However, FIG. 13 diagrams analternative arrangement whereby only a lower segment 168 of the interiorof a roller drum 170 is used as an accumulator to eliminate the need forgroundactuated valving for each foot 172. Feet 172 approximate the formof the foot shown in FIG. 7. Accumulator segment 168 does not rotatewith the drum, but instead contacts the inner surface of the drum atsliding air seals 174 attached to the ends of segment 168. Air issupplied to the accumulator from an air compressor 176 mounted oncompactor frame 177 through a hose 178 that directs air into a hollowportion of a drum axle 179 at a swivel connection 180. Air injected intothe axle enters the segment through openings in the axle. With theaccumulator extending through the are shown and with the roller movingfrom right to left as indicated by the arrow, air enters each compactionfoot from a time just after the foot enters the soil to a short timeafter the foot leaves the soil, thereby blowing dirt from each foot tokeep the air passages clear. However, the roller could be operated inthe reverse direction, in which event air would be injected into eachfoot and directed toward the soil beginning from a time just beforee'ach foot enters the soil and continuing until just before each footleaves the soil. In either case, a compactive effect greater than thatobtainable through use of the roller without air injection would beobtained in air permeable soils.

FIG. 14 discloses an arrangement similar to that of FIG. 13, butincorporated in a plain roller drum 182 having no tamping feet, buthaving air passages 184 through the periphery of the drum. An airchamber segment 186 within the drum remains stationary as the drumrotates so that only the downwardly directed air passages will receiveair. The accumulator may be supplied with air from a compressor on theframe in the manner shown in FIG. 13. Normally roller 182 would rollfrom right to left as shown, but as discussed with respect to FIG. 13,the desired compactive effect would also be obtained when operating theroller in reverse.

It is important to point out that both the roller of FIG. 13 and that ofFIG. 14 may be of either the static or vibratory type. If of the lattertype, such a roller could incorporate any of the common types ofvibrations generating means, such as, for example, an eccentric, drivenshaft extending through the center of the roller drum as shown in UnitedStates Patent 3,203,201, to Harbke, or one having a driven rotaryeccentric weight as shown in United States Patent 2,025,703, to Baily etal.

FIGS. 15 and 16 diagram an alternate arrangement for supplying air froman external air supply 188 on the frame 189 of a compaction rollerhaving a drum 190, to headers 192 within the drum extending parallel tothe cylindrical surface of the drum, along each longitudinal row oftamping feet 194. Each header has a series of laterals 196 which extendinto the feet 194 to supply the latter with air. Air is supplied to eachheader when the header rotates with the drum to a downward position,through a pair of air hoses 197 from the compressor 188. The hosesconnect to injection heads 198, 199 mounted by struts 200 from framemembers 201, 202 in stationary positions at the lower opposite end wallsof the drum. End openings of the headers come successively into registerwith the injection heads as the drum rotates. The heads are maintainedunder constant internal air pressure at the drum surfaces by slidingseals 204 so that the headers are successively injected with air as theyregister with the heads.

FIGS. 17 and 18 disclose an alternative arrangement for supplyingpressurized air to tamping feet 210 on a compaction roller drum 212 whenthe feet roll into predeterimned positions with respect to the ground.Referring first to FIG. 18, compressed air from a compressor (not shown)enters a drilled passage 213 within one end of an axial drum shaft 214at an end connection 215. Shaft 214 is fixed against rotation relativeto a compactor frame 216 by a key 217 in a bearing member 218 carried bythe frame.

Air passes through drilled passage 213 and enters the inside of drum 212at an opening 220. A rotary seal 222 seals the drum against leakage ofair at the shaft. Air within the drum enters a valve housing 224 throughan orifice 226. If desired, a valve (not shown) operable from outsidethe drum could be placed at orifice 226 to retain accumulated airpressure within the drum during periods of shutdown. However, so long asthe compressor is operating, high pressure air is maintained withinchamber 228 of the valve housing.

A sleeve 230 slidably surrounds valve housing 224 and is sealed againstair leakage by O-rings 225. The sleeve is connected to a system ofradial tubes 232 which lead into the interiors of hollow tamping feet210 of one peripheral row of such feet. Longitudinal feeder pipes 234connected to radial tubes 232 direct air into the remaining peripheralrows of feet 210 through short radial pipe sections 236.

Ports 238, 240, as shown in FIG. 17, are placed at predeterminedlocations in sleeve 230 to provide clear air passage between valvechamber 228 and only those tamping feet 210 which are connected directlyor indirectly with radial tubes in register with such ports. With theillustrated arrangement, air is supplied only to the three tamping feet210 of each peripheral row which are in contact with the soil to injectair into the soil and to the single tamping foot of each peripheral rowwhich is in its uppermost position so as to blow accumulated soil fromthe perforate insert 242 at the soil contacting surface of each foot.

FIG. 19 shows a peripheral shell portion 244 of a plain roller similarto the roller of FIGS. 17 and 18 except for the absence of tamping feeton the former. The valving and air supply arrangements of the tworollers are identical. The major difference is that the drum shell 244is provided with air injection orifices 246, and porous inserts 248 areset within the orifices. A housing 250- attached to the inner face ofshell 244 receives radial air supply tube 252 corresponding to tubes 232of FIG. 18 leading from the primary valve housing and distributes airinto orifice 246.

FIG. 20 illustrates a hand operated vibratory tamper 260 having atamping plate 262 with a porous air injection insert 264 in the soilcontacting surface 265 thereof. Air is supplied to the plate through ahose 266 extending from the plate upwardly along a handle 267 and to anair compressor (not shown). A manually operated valve 268- on the handlecontrols the admission of air to the plate. A motor driven vibrator 269is mounted on the plate. If desired, an air driven vibrator could besubstitued for the motor driven vibrator shown, in which case air wouldfirst be directed through the hose to the vibrator and then exhaustedfrom the vibrator into the tamping plate.

FIG. 21 illustrates diagrammatically the application of the presentmethod to a compaction vehicle 270 mounting at its forward end aplurality of air driven vibrators 272 from which depend a series ofcompaction feet 274. An air line 276 extends from a compressor on thevehicle to the vibrators to drive the same, and air is exhausted fromthe vibrators into the tamping feet.

From the foregoing apparatus, it will be apparent that an air pressurecan be applied to the soil which is independent of the mechanicalcontact pressure applied by the foot, roller or tamper, and that the airpressure can be varied to meet the requirements of varying soilconditions. The applied air pressure, being supplied from a sourceexternal to the soil, is also independent of the porosity of the soil,and thus again the applied air pressure can be varied to meet changingneeds.

Having illustrated several means for carrying out the method, it shouldbe obvious to those skilled in the art that the method and apparatus ofthe present invention are capable of modification in arrangement anddetail. I claim as my invention all such modifications as come withinthe true spirit and scope of the following claims.

I claim:

1. A method of compacting a permeable mass of particulate materialhaving initial pore gas pressures acting between the particles of saidmass,

said method comprising:

injecting a gas into said mass under a pressure sufficient to increasesaid pore gas pressures to a level greater than said initial pore gaspressures,

while said pore gas pressures are increased, applying a mechanicalcontact pressure to a surface portion of said mass,

controlling the injection pressure of said gas at a level suflicientlyhigh to densify said mass and sufficiently low to avoid the quick stateof said mass,

continuing the application of said mechanical contact pressure duringthe densification of said mass.

2, A method according to claim -1 including inducing the bleeding of theinjected gas from said mass by continuing the application of saidmechanical contact pressure until the mass is densified and at leastuntil the applied gas pressure is relieved.

3. A method according to claim 1 including applying the mechanicalcontact pressure before injecting the gas into said pores and includinginjecting said gas at a pressure producing positive intergranularpressures within said mass at levels above a predetermined minimumpositive intergranular pressure so as to avoid inducing a quick statewithin said mass.

4. A method according to claim 1 including continuing said injection ofgas into said mass during the densification of said mass whereby saidpore gas pressures are maintained at a level above said initial pore gaspressures throughout substantially the entire period during whichdensification of said mass occurs.

5. A method according to claim 1 wherein the injected gas is chemicallynonreactive with said particulate material.

6. A method according to claim 1 wherein the pressure of the injectedgas is independent of said mechanical contact pressure.

7. A method according to claim 1 wherein said gas is injected into saidmass from a position at said surface portion.

8. A method according to claim 7 wherein said gas is injected into saidmass at points spaced inwardly of the outer peripheral limits of saidsurface portion.

9. A method of compacting a plot of ground comprising: I

applying a mechanical contact pressure to the surface of a selected zoneof said plot,

while said mechanical surface contact pressure is being applied,injecting gas into the ground within said zone at a pressuresufficiently high to effect a collapse thereof but sufliciently low tomaintain said ground surface in a load-supporting condition during saidcollapse,

and while said ground is collapsing, maintaining said mechanical contactpressure against said surface.

10. A method according to claim 9 wherein the application of saidcontact pressure and injection of gas simultaneously as aforesaid isrepeated at different zones of said plot until the entire said plot iscompacted.

11. A method according to claim 9 wherein the gas is injected from asource external to said ground and at a pressure independent of theambient gas pressure and mechanical contact pressure.

12. Earth compacting apparatus comprising in coma bination:

pressure applying means for applying a mechanical contact pressure tothe surface of a localized zone of earth,

gas injection means for injecting gas under pressure into the earthwithin said zone during the application of mechanical contact pressureby said pressure applying means,

said gas injection means including means for controlling the appliedpressure of said injected gas at a level sufficiently high to promotedensification of said 1 1 zone and sufiiciently low to maintain thesurface of said zone in a load supporting condition.

13. Apparatus according to claim 12 wherein said pressure applying meansincludes means for maintaining a contact pressure on the surface of saidzone during the settlement of earth within said zone and said gasinjection means includes means for continuing the injection of said gasinto said zone during said settlement.

14. Apparatus according to claim 12 including means for controlling theapplication of said gas pressure and said contact pressure in accordancewith a predetermined sequence of time.

15. Apparatus according to claim 12 wherein said pressure applying meansincludes an earth compaction roller and said gas injection meansincludes orifice means embodied in a peripheral portion of said roller.

16. Apparatus according to claim 12 wherein said pressure applying meansincludes a compaction foot means having an earth contacting surface andsaid gas injection means includes orifice means in said foot means andspaced inwardly of the periphery of said surface.

17. Apparatus according to claim 16 wherein said orifice means includesplural orifices distributed over said earth contacting surface.

18. Apparatus according to claim 16 wherein said orifice means includesan air permeable portion of said earth contacting surface.

19. Apparatus according to claim 12 including vibration generating meansfor vibrating said zone while applying said mechanical contact pressureand while injecting said gas.

20. Apparatus according to claim 12 wherein:

said pressure applying means includes an earth contacting surface, saidgas injection means includes an opening adjacent said earth contactingsurface in communication with a source of gas under pressure,

and means for cleaning soil particles from said opening by blowing gasfrom said source outwardly through said opening when said opening is outof engagement with said earth.

21. Apparatus for compacting a porous mass of particulate materialcomprising:

means for intruding a gas under pressure into said mass,

means for applying a mechanical contact pressure to a surface portion ofthe same said mass to compress said mass and for maintaining saidcontact pressure during the injection of said gas,

and means for controlling the pressure at which said gas is applied at alevel sufficiently high to increase the pore gas pressures Within saidmass while said mass is under compression but sufficiently low tomaintain positive intergranular pressures within said mass.

22. Apparatus according to claim 21 wherein said means for injecting gasunder pressure includes control means for varying said pressure.

23. A method of compacting soil in place comprising:

applying a downward mechanical contact pressure to a surface area of aselected zone of said soil to be compacted, to place the soil Withinsaid zone under compression, said applied pressure being at a levelbelow that required to penetrate said surface area,

and while said zone is under compression, intruding a pressurized gasinto the soil within said zone under a pressure sufiiciently high toeffect a settling of the soil within said zone and below that pressurerequired to place said soil in its quick state,

and maintaining said zone under compression during the settling of saidsoil.

24. A method of compacting a porous unsaturated mass of particulatematerial having an initial volume and initial pore gas pressures andinitial positive intergranular pressures,

said method comprising:

applying a mechanical surface contact pressure to a surface area of saidmass while restraining the remaining peripheral areas of said massagainst any substantial bodily displacement so as to place said mass incompression,

during the application of said mechanical contact pressure to saidsurface area, intruding gas into said mass in compression under apressure sufiicient to increase the pore gas pressures of said mass andbelow that pressure required to reduce the intergranular pressures tozero,

continuing the simultaneous application of contact pressure to saidsurface area and the intrusion of said gas to shrink the volume of saidmass,

and continuing the application of said contact pressure and theintrusion of said gas during said shrinkage.

References Cited UNITED STATES PATENTS 1,117,333 11/1914 Cooper 94-442,384,469 9/1945 Kalix 9448 1,952,162 3/1934 Gee 6136 2,719,029 9/1955Steuerman 61-36 XR 2,866,422 12/ 1958 Colson 1116 2,975,735 3/1961Purvance 111-6 3,029,756 4/ 1962 Krivda 1l1--6 3,269,039 8/1966 Bodine17240 XR JACOB L. NACKENOFF, Primary Examiner US. Cl. X.R.

